The present invention relates to a facsimile transmission process and apparatus utilizing data compression and buffer memory means.
A facsimile apparatus to which the present invention relates comprises a scanner having a charge coupled device (CCD) photosensor array. The array is continuously scanned to produce data signals representing scan lines which are fed to a buffer memory for temporary storage. The operation is such that the array is scanned in response to one horizontal scan pulse and the resulting data signals shifted out of the array to the buffer memory in response to a next horizontal scan pulse.
A compression unit typically performing run length compression reads the data out of the buffer memory and compresses the data to increase the transmission speed. The compression speed varies in accordance with the amount of actual information in the data. For example, if a scan line consists of entirely blank data, the compression time will be very short. However, if the scan line represents a complex pattern, the compression time will be long. For this reason, the buffer memory is provided to store at least two horizontal scan lines in such a manner that one line may be read out for compression while another scan line is being stored from the scanner.
In a CCD array the scan rate must be kept constant in order to prevent a variation in the integration time of internal capacitors which would produce erroneous data. Thus, the buffer memory is operated in a demand manner. More specifically, data is read out of the buffer memory upon demand from the compressor in accordance with the instantaneous compression rate. When enough vacant space becomes available in the buffer memory to store one scan line, a scan line from the scanner is stored in the buffer memory and a vertical scan or incrementation operation is performed. Where the compression rate is low, data will not be stored in the buffer memory in response to each horizontal scan and some horizontal scans will be skipped. For this reason, a vertical scan operation is performed only when a line of data is to be stored in the buffer memory.
When the compression rate is low and a previous horizontal scan has been skipped, the next line of data stored in the buffer memory will correspond to a horizontal scan performed while a vertical scan was not being performed, or while the array was stationary relative to an original document. Thus, the area of the document represented by the horizontal scan will be narrow.
On the contrary, when the compression rate is high and no scan lines are skipped, vertical scan will be performed continuously. Thus, the area of the document represented by each horizontal scan will be wider.
In an apparatus of the present type, a light image of a linear portion of the original document is focussed onto the photosensor array. In such an array, light incident on photosensor elements of the array causes accumulation of charge in proportion to incident light intensity and time. The output signals of the elements correspond to the accumulated charge. The array also comprises an analog shift register into which the output signals of the elements are transferred in parallel. Horizontal clock pulses cause the signals to be serially fed out of the shift register.
This operation constitutes a horizontal scan of the light image of the document. During the horizontal scan the document is moved by one increment perpendicular to the array to effect vertical scan.
While the basic prior art apparatus of this type is generally feasible in practical application, a certain amount of distortion is encountered when the apparatus is employed in conjunction with a data compression unit such as a run lenth encoder. Such a compression unit operates at variable speed which depends on the original document. Where the document contains a large proportion of dark areas, the compression process is relatively slow. For this reason, it is often necessary to interrupt scanning to prevent the data fed from the array into the compression unit or encoder from exceeding the capacity of the encoder.
When the array is controlled by the encoder to begin or resume scanning, the array is scanned simultaneously with moving the document for vertical scan. As a result, the area convered in the vertical direction by each photosensor element for the current scan line corresponds only to the area of the photosensor element. This differs greatly from continuous scanning where the area covered in the vertical direction by each element corresponds to the product of the width of the element in the horizontal direction and the distance the document is moved relative to the array plus the area of the element. This is because when scanning is begun the output signals of the elements are immediately transferred to the shift register and thereafter fed out. The document is at rest when the transfer is made. Thus, not only is the accumulated charge different from that in continuous scan but image areas between scan lines are not scanned.
A typical prior art expedient to overcome this problem is disclosed in U.S. Pat. No. 4,081,843 in which each horizontal scan operation is performed while the document is stationary relative to the array. Although distortion is eliminated to the extent that the scan line density is maintained constant, the problem is not satisfactorily overcome since the scanning speed is limited by the necessity to stop the vertical scan between each two scan lines and the fact that information between adjacent vertical scan lines is lost.
Another method of overcoming this problem is disclosed in copending U.S. patent application Ser. No. 906,107, filed May 15, 1978 now U.S. Pat. No. 4,217,611 which is assigned to the same assignee as the present application. In accordance with this prior method, delay means delay horizontal scanning relative to incrementation movement of the document for vertical scan to ensure that all scanning is performed after the document has moved. Thus, the charge accumulated from scanning all lines is similar regardless of whether the scanning is performed continuously or resumed after a temporary interruption necessitated by a full condition of a buffer memory. The present invention constitutes a significant improvement over the prior method in that it eliminates the necessity to delay the horizontal scan and the delay means required to perform said function while maintaining the vertical scan line density constant.